Your "social utility" argument only works if you take into account all factors, which you inevitably will not be able to do. The most important one, in my experience, is the ability to obtain work experience at a young age that will enable you to climb the economic ladder. My first job paid only $6/hr, and you can bet your ass that I was not worth even that much to the employer. I didn't yet know how to work. Most modern kids, unless they grew up on a farm, will likewise not be very valuable to their employers for the first few months of their employment.
Was I taken advantage of at 16-years-old when my employer paid me $6/hour? Of course not. No one would argue that I was. So why is that now illegal? Look at the youth unemployment rates, they're astronomical. 15.1 percent for young men actively looking for work, 24.8 percent for blacks, 16.5 percent for hispanics. Source:
http://www.bls.gov/news.release/youth.nr0.htm
That $6/hour job enabled me to get hired for better paying positions. The economics behind it is that I learned how to 'work' at the $6/hour job, and thus become worth more per hour. As you correctly mentioned, few low-paying jobs will ever allow you to move up to a higher-paying wage. What you did not mention, however, was that you can now far more easily get a higher paying position than previously. I went from $6/hour, to $8/hour, to $10/hour, to $12/hour, to (variable) around $15/hour, to my previous career wage of around $40/hour. That number keeps rising, and will take another massive jump after medical school and residency. The foundation for much of that was my original $6/hour job.